Method of enhancing transfer of coagulated colloid onto a substrate during electrocoagulation printing

Abstract

An improved electrocoagulation printing method comprising the steps of (a) providing a positive electrode formed of an electrolytically inert metal and having a continuous passivated surface moving at substantially constant speed along a predetermined path, the passivated surface defining a positive electrode active surface; (b) forming on the positive electrode active surface a plurality of dots of colored, coagulated colloid representative of a desired image, by electrocoagulation of an electrolytically coagulable colloid present in an electrocoagulation printing ink comprising a liquid colloidal dispersion containing the electrolytically coagulable colloid, a dispersing medium, a soluble electrolyte and a coloring agent; and (c) bringing a substrate into contact with the dots of colored, coagulated colloid to cause transfer of the colored, coagulated colloid from the positive electrode active surface onto the substrate and thereby imprint the substrate with the image. The improvement resides in maintaining the positive electrode active surface and the ink at a temperature of about 35° C. to about 60° C. to increase the viscosity of the coagulated colloid in step (b) so that the dots of colored, coagulated colloid remain coherent during their transfer in step (c), thereby enabling the colored, coagulated colloid to be substantially completely transferred onto the substrate in step (c).

I claim: 
     
       1. In an electrocoagulation printing method comprising the steps of: a) providing a positive electrode formed of an electrolytically inert metal and having a continuous passivated surface moving at a constant speed along a selected path, said passivated surface defining a positive electrode active surface;   b) forming on said positive electrode active surface a plurality of dots of colored, coagulated colloid representative of a desired image, by electrocoagulation of an electrolytically coagulable colloid present in an electrocoagulation printing ink comprising a liquid colloidal dispersion containing said electrolytically coagulable colloid, a dispersing medium, a soluble electrolyte and a coloring agent; and   c) bringing a substrate into contact with the dots of colored, coagulated colloid to cause transfer of the dots of colored, coagulated colloid from the positive electrode active surface onto said substrate and to imprint said substrate with said image; the improvement which comprises maintaining said positive electrode active surface and said ink at a temperature of about 35° C. to about 60° C. to increase viscosity of the coagulated colloid in step (b) said that the dots of colored, coagulated colloid remain coherent during transfer in step (c) to enable the colored, coagulated colloid to be substantially completely transferred onto said substrate in step (c).     
     
     
       2. A method as claimed in claim 1, wherein the temperature of said positive electrode active surface and said ink is about 40° C. 
     
     
       3. A method as claimed in claim 1, wherein said ink is maintained at said temperature by heating said positive electrode active surface and applying said ink on the heated electrode surface to cause a transfer of heat therefrom to said ink. 
     
     
       4. A method as claimed in claim 1, wherein said dispersing medium is water and said electrolyte is selected from the group consisting of alkali metal halides and alkaline earth metal halides. 
     
     
       5. A method as claimed in claim 4, wherein said electrolyte is present in said ink in an amount of about 4.5 to about 6% by weight, based on the total weight of the ink. 
     
     
       6. A method as claimed in claim 5, wherein said electrolyte is potassium chloride. 
     
     
       7. A method as claimed in claim 4, wherein said substrate is a water absorbent paper. 
     
     
       8. A method as claimed in claim 7, wherein said water absorbent paper has a thickness of about 60 μm to about 70 μm. 
     
     
       9. A method as claimed in claim 1, wherein steps (b) and (c) are repeated several times to define a corresponding number of printing stages arranged at selected locations along said path and each using a coloring agent of different color, and to produce several differently colored images of coagulated colloid which are transferred at respective transfer positions onto said substrate in superimposed relation to provide a polychromic image. 
     
     
       10. A method as claimed in claim 9, wherein said positive electrode is a cylindrical electrode having a central longitudinal axis and rotating at said constant speed about said longitudinal axis, and wherein said printing stages are arranged around said positive cylindrical electrode. 
     
     
       11. A method as claimed in claim 10, wherein step (b) is carried out by: i) providing a plurality of negative electrolytically inert electrodes electrically insulated from one another and arranged in rectilinear alignment to define a series of corresponding negative electrode active surfaces disposed in a plane parallel to the longitudinal axis of said positive electrode and spaced from the positive electrode active surface by a constant selected gap, said negative electrodes being spaced from one another by a distance at least equal to said electrode gap;   ii) coating the positive electrode active surface with an olefinic substance and a metal oxide to form on said surface micro-droplets of olefinic substance containing the metal oxide;   iii) filling said electrode gap with said electrocoagulation printing ink;   iv) electrically energizing selected ones of said negative electrodes to cause point-by-point selective coagulation and adherence of the colloid onto the olefin and metal oxide-coated positive electrode active surface opposite the electrode active surfaces of said energized negative electrodes while said positive electrode is rotating to form said dots of colored, coagulated colloid; and   v) removing any remaining non-coagulated colloid from said positive electrode active surface.   
     
     
       12. A method as claimed in claim 11, wherein step (b) (ii) is carried out by providing a distribution roller extending parallel to said positive electrode and having a peripheral coating comprising an oxide ceramic material, applying said olefinic substance in the form of an oily dispersion containing said metal oxide as dispersed phase onto the ceramic coating to form on a surface thereof a film of said oily dispersion uniformly covering the surface of said ceramic coating, said film of oily dispersion breaking down into micro-droplets containing said olefinic substance in admixture with said metal oxide and having substantially uniform size and distribution, and transferring said micro-droplets from said ceramic coating onto said positive electrode active surface. 
     
     
       13. A method as claimed in claim 12, wherein said oxide ceramic material comprises a fused mixture of alumina and titania. 
     
     
       14. A method as claimed in claim 12, wherein said oily dispersion is applied onto said ceramic coating by disposing an applicator roller parallel to said distribution roller and in pressure contact engagement therewith to form a first nip, and rotating said applicator roller and said distribution roller in register while feeding said oily dispersion into said first nip, such that said oily dispersion upon passing through said first nip forms said film uniformly covering the surface of said ceramic coating. 
     
     
       15. A method as claimed in claim 14, wherein said micro-droplets are transferred from said distribution roller to said positive electrode by disposing a transfer roller parallel to said distribution roller and in contact engagement therewith to form a second nip, positioning said transfer roller in pressure contact engagement with said positive electrode to form a third nip, and rotating said transfer roller and said positive electrode in register for transferring said micro-droplets from said distribution roller to said transfer roller at said second nip and thereafter transferring said micro-droplets from said transfer roller to said positive electrode at said third nip. 
     
     
       16. A method as claimed in claim 15, wherein said applicator roller and said transfer roller are each provided with a peripheral covering of a resilient material which is resistant to attack by said olefinic substance. 
     
     
       17. A method as claimed in claim 11, wherein step (b) (ii) is carried out by providing first and second distribution rollers extending parallel to said positive electrode and each having a peripheral coating comprising an oxide ceramic material, applying said olefinic substance in the form of an oily dispersion containing said metal oxide as dispersed phase onto the ceramic coating of said first distribution roller to form on a surface thereof a film of said oily dispersion uniformly covering the surface of said ceramic coating, said film of oily dispersion at least partially breaking down into micro-droplets containing said olefinic substance in admixture with said metal oxide and having substantially uniform size and distribution, transferring the at least partially broken film from said first distribution roller to said second distribution roller to cause said film to substantially completely break on the ceramic coating of said second distribution roller into said micro-droplets having substantially uniform size and distribution, and transferring said micro-droplets from the ceramic coating of said second distribution roller onto said positive electrode active surface. 
     
     
       18. A method as claimed in claim 17, wherein the ceramic coatings of said first distribution roller and said second distribution roller comprise the same oxide ceramic material, and wherein said oxide ceramic material comprises a fused mixture of alumina and titania. 
     
     
       19. A method as claimed in claim 17, wherein said oily dispersion is applied onto the ceramic coating of said first distribution roller by disposing an applicator roller parallel to said first distribution roller and in pressure contact engagement therewith to form a first nip, and rotating said applicator roller and said first distribution roller in register while feeding said oily dispersion into said first nip, such that said oily dispersion upon passing through said first nip forms said film uniformly covering the surface of said ceramic coating. 
     
     
       20. A method as claimed in claim 19, wherein said at least partially broken film of oily dispersion is transferred from said first distribution roller to said second distribution roller and said micro-droplets are transferred from said second distribution roller to said positive electrode by disposing a first transfer roller between said first distribution roller and said second distribution roller in parallel relation thereto, positioning said first transfer roller in pressure contact engagement with said first distribution roller to form a second nip and in contact engagement with said second distribution roller to form a third nip, rotating said first distribution roller and said first transfer roller in register for transferring said at least partially broken film from said first distribution roller to said first transfer roller at said second nip, disposing a second transfer roller parallel to said second distribution roller and in pressure contact engagement therewith to form a fourth nip, positioning said second transfer roller in pressure contact engagement with said positive electrode to form a fifth nip, and rotating said second distribution roller, said second transfer roller and said positive electrode in register for transferring said at least partially broken film from said first transfer roller to said second distribution roller at said third nip, then transferring said micro-droplets from said second distribution roller to said second transfer roller at said fourth nip and thereafter transferring said micro-droplets from said second transfer roller to said positive electrode at said fifth nip. 
     
     
       21. A method as claimed in claim 20, wherein said applicator roller, said first transfer roller and said second transfer roller are each provided with a peripheral covering of a resilient material which is resistant to attack by said olefinic substance. 
     
     
       22. A method as claimed in claim 11, further including the step of polishing the olefin and metal oxide-coated positive electrode active surface to increase adherence of said micro-droplets onto said positive electrode active surface, prior to step (b) (iii) of each printing stage. 
     
     
       23. A method as claimed in claim 11, wherein said olefinic substance is selected from the group consisting of arachidonic acid, oleic acid, linoleic acid, linolenic acid, palmitoleic acid, corn oil, linseed oil, olive oil, peanut oil, soybean oil and sunflower oil, and wherein said metal oxide is selected from the group consisting of aluminum oxide, ceric oxide, chromium oxide, cupric oxide, magnesium oxide, manganese oxide, titanium dioxide and zinc oxide. 
     
     
       24. A method as claimed in claim 23, wherein said metal oxide is present in said oily dispersion in an amount of about 15 to about 40% by weight, based on the total weight of the dispersion. 
     
     
       25. A method as claimed in claim 23, wherein said olefinic substance is oleic acid or linoleic acid and said metal oxide is chromium oxide. 
     
     
       26. A method as claimed in claim 25, wherein said oily dispersion contains about 75 wt. % of oleic acid or linoleic acid and about 25 wt. % of chromium oxide. 
     
     
       27. A method as claimed in claim 10, wherein the temperature of said positive electrode active surface and said ink is about 40° C. 
     
     
       28. A method as claimed in claim 10, wherein said ink is maintained at said temperature by heating said positive electrode active surface and applying said ink on the heated electrode surface to cause a transfer of heat therefrom to said ink. 
     
     
       29. A method as claimed in claim 10, wherein said dispersing medium is water and said electrolyte is selected from the group consisting of alkali metal halides and alkaline earth metal halides. 
     
     
       30. A method as claimed in claim 29, wherein said electrolyte is present in said ink in an amount of about 4.5 to about 6% by weight, based on the total weight of the ink. 
     
     
       31. A method as claimed in claim 30, wherein said electrolyte is potassium chloride. 
     
     
       32. A method as claimed in claim 29, Wherein said substrate is a water absorbent paper. 
     
     
       33. A method as claimed in claim 32, wherein said water absorbent paper has a thickness of about 60 μm to about 70 μm. 
     
     
       34. A method as claimed in claim 10, wherein said substrate is in the form of a continuous web which is passed through said respective transfer positions for being imprinted with said colored images at said printing stages. 
     
     
       35. A method as claimed in claim 34, wherein step (c) is carried out by providing at each transfer position a pressure roller extending parallel to said positive electrode and in pressure contact engagement therewith to form a nip and permit said pressure roller to be driven by said positive electrode upon rotation thereof, and guiding said web to pass through said nip. 
     
     
       36. A method as claimed in claim 35, wherein each said pressure roller is provided with a peripheral covering of a synthetic rubber material. 
     
     
       37. A method as claimed in claim 36, wherein said synthetic rubber material comprises a polyurethane having a Shore A hardness of about 95. 
     
     
       38. A method as claimed in claim 10, further including the step of removing after step (c) of each printing stage any remaining coagulated colloid from said positive electrode active surface. 
     
     
       39. A method as claimed in claim 38, wherein said positive electrode is rotatable in a selected direction and wherein any remaining coagulated colloid is removed from said positive electrode active surface by providing an elongated rotatable brush extending parallel to the longitudinal axis of said positive electrode, said brush being provided with a plurality of radially extending bristles having extremities contacting said positive electrode active surface, rotating said brush in a direction opposite to the direction of rotation of said positive electrode to cause said bristles to frictionally engage said positive electrode active surface, and directing jets of cleaning liquid under pressure against said positive electrode active surface, from either side of said brush. 
     
     
       40. A method as claimed in claim 39, wherein said positive electrode active surface and said ink are maintained at said temperature by heating said cleaning liquid to heat said positive electrode active surface upon contacting same and applying said ink on the heated electrode surface to cause a transfer of heat therefrom to said ink.